@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Science, Faculty of"@en, "Earth, Ocean and Atmospheric Sciences, Department of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Narod, B. Barry"@en ; dcterms:issued "2010-01-28T21:18:12Z"@en, "1975"@en ; vivo:relatedDegree "Master of Science - MSc"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description """For determining the thickness of ice, radio echo sounding of glaciers is well established as a technique for rapid gathering of data. However it has become evident that radio echo sounder parameters must be tailored to meet specific requirements in order to achieve best results. In particular rapid sounding of temperate, alpine glaciers and larger polar valley glaciers could be surveyed by a radio echo sounder having a very short pulse, very wide land response and narrow beam antenna. Such requirements can be fulfilled by Ultra High Frequency (300 MHz - 3 GHz) radio echo sounders, improved performance being achieved at the expense of decreased maximum range. This thesis, after considering previous attempts at radio echo sounding of glaciers, with respect to surveying valley and temperate glaciers, proposes and details a UHF radio echo sounder operating at 840 MHz. Conventional performance is predicted, and some new experiments, possible because of the short wavelength, are proposed. Appended to the thesis is a review and discussion of problems associated with the use of thermistors as thermometers in snow or ice. A procedure is described for optimizing the techniques of thermistor selection and use."""@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/19268?expand=metadata"@en ; skos:note "ULTRA HIGH FREQUENCY RADIO ECHO SOUNDING OF GLACIERS V by B. Barry Narod B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOB THE DEGREE OF MASTER OF SCIENCE i n the Department of Geophysics and Astronomy We accept t h i s t h e s i s as conforming to the r e q u i r e d standard The U n i v e r s i t y of B r i t i s h Columbia A p r i l 1975 In present ing th is thes is in p a r t i a l fu l f i lment o f the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make i t f r ee ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by h is representa t ives . It is understood that copying or p u b l i c a t i o n of th is thes is fo r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permiss ion. Depa rtment The Un ivers i ty of B r i t i s h Columbia 20 75 Wesbrook Place Vancouver, Canada V6T 1W5 i ABSTRACT For determining the t h i c k n e s s of i c e , r a d i o echo sounding of g l a c i e r s i s w e l l e s t a b l i s h e d as a technigue f o r r a p i d g a t h e ring of data. However i t has become evident t h a t r a d i o echo sounder parameters must be t a i l o r e d to meet s p e c i f i c requirements i n order to achieve best r e s u l t s . In p a r t i c u l a r r a p i d sounding of temperate, a l p i n e g l a c i e r s and l a r g e r p o l a r v a l l e y g l a c i e r s c o u l d be surveyed by a r a d i o echo sounder having a very sho r t p u l s e , very wide land response and narrow beam antenna. Such requirements can be f u l f i l l e d by U l t r a High Frequency (300 MHz - 3 GHz) r a d i o echo sounders, improved performance being achieved at the expense of decreased maximum range. T h i s t h e s i s , a f t e r c o n s i d e r i n g previous attempts a t r a d i o echo sounding of g l a c i e r s , with res p e c t to s u r v e y i n g v a l l e y and temperate g l a c i e r s , proposes and d e t a i l s a UHF r a d i o echo sounder o p e r a t i n g a t 840 MHz. Conventional performance i s p r e d i c t e d , and some new experiments, p o s s i b l e because of the sh o r t wavelength, are proposed. Appended to the t h e s i s i s a review and d i s c u s s i o n of problems a s s o c i a t e d with the use of t h e r m i s t o r s as thermometers i n snow or i c e . A procedure i s d e s c r i b e d f o r o p t i m i z i n g the techniques of t h e r m i s t o r s e l e c t i o n and use. i i TABLE OF CONTENTS Abstract . . . . . . . I L i s t Of Tables Iv L i s t Of Figures V Acknowledgements Vi Chapter 1: Introduction 1 1.1 Background 1 1.2 D i e l e c t r i c Propert ies Of Ice 4 Chapter 2: System Design 6 2.1 General Descr ipt ion . . . 6 2.2 Se lect ion Of The C a r r i e r Frequency 7 2.3 System Performance 8 Range 10 2.4 Antenna Design . . . . . . . . . 1 2 Chapter 3: C i r c u i t Design 15 3.1 Transmitter C i r c u i t 17 3.2 Receiver/pulse Generator C i r c u i t . , .18 3.3 Display And Recorder . . . . . . . . . . . . . . . 2 1 Chapter 4: System C a p a b i l i t i e s . . . 2 5 4.1 System Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5 4.2 Maximum Range And Besolution 26 4.3 R e f l e c t i v i t y o f Layers H i t h i n A Glac i e r . . . . 2 8 4.4 P o l a r i z a t i o n 29 4.5 Conclusion 34 Bibliography 36 Appendix 45 i i i 1 I n t r o d u c t i o n ....46 2 Thermistor C h a r a c t e r i s t i c s , 48 3 Bridges And Bridge Power Supply ......52 4 Transmission Line E f f e c t s .....61 5 Evaluation Of A v a i l a b l e Systems (dmnus) ............62 Conclusion 64 Bi b l i o g r a p h y 67 LIST OF TABLES i v 0 Table 2.1: Radar Parameters .7 Table 2.2: System Performance 10 Table 1: D i g i t a l Multimeters ......63 Table 2: D i g i t a l Multimeter E r r o r s ....64 LIST OF FIGURES Figure 2.1 .. 13 Figure 3.1: Radar Interconnection Diagram .16 Figure 3.2: Transmitter Block Diagram . .....17 Figure 3.3: Receiver Block Diagram 19 Figure 3.4: I n t e n s i t y Modulation Recording Block Diagram .23 Figure 3.5: D i g i t i z e r Block Diagram ....24 Figure 1: Generalized Bridge .............................47 Figure 2: Bridge C o n f i g u r a t i o n s ......54 Figure 3: D i g i t a l Multimeter E r r o r s ...64 v i ACKNOWLEDGEMENTS I wish to thank my s u p e r v i s o r , Dr. Garry C l a r k e , f o r h i s patience and guidance during the course of t h i s work. I would a l s o l i k e t o thank Dr. C l a r k e , Dr. R, B. R u s s e l l , the Department of Geophysics and Astronomy, and the Department of Physics f o r f i n a n c i a l support I r e c e i v e d while I was completing t h i s t h e s i s . I am very g r a t e f u l to Dr. Ron Goodman of Calgary f o r h i s i n v a l u a b l e a s s i s t a n c e and advi c e while development of the r a d i o echo sounder progressed. I wish t o acknowledge the s e r v i c e s of the U n i v e r s i t y ' s Computing Center, which supported t h i s t y p e s c r i p t ; and the numerous manufacturers who p a r t i c i p a t e d i n the development of the r a d i o echo sounder. I thank the Department of Geophysics and Astronomy f o r p r o v i d i n g l a b o r a t o r y f a c i l i t i e s f o r t h i s p r o j e c t . I thank a l l of my f r i e n d s among the f a c u l t y , s t a f f and students of the Department, e s p e c i a l l y Mr. Peter Michelow, f o r a l l t h e i r c o n t r i b u t i o n s to t h i s p r o j e c t . F i n a l l y , and e s p e c i a l l y , I wish to thank the N a t i o n a l Research C o u n c i l of Canada which f i n a n c e d the development of both the UHF r a d i o echo sounder and the t h e r m i s t o r r e s i s t a n c e bridge. The r a d i o echo sounder was fi n a n c e d by grant A3479 from the N a t i o n a l Research C o u n c i l to Dr. C l a r k e . The r e s i s t a n c e bridge was f i n a n c e d by grant A4327 from the v i i N a t i o n a l Research C o u n c i l c f Canada a l s o to Dr. C l a r k e . 1 CHAPTER 1 : INTRODUCTION 1.1 BACKGROUND Radio echo sounding of i c e was \" s u c c e s s f u l \" as e a r l y as 1946 when a i r c r a f t p i l o t s using pulsed radar a l t i m e t e r s ever A n t a r c t i c i c e r e p o r t e d p o s s i b l e f a t a l e r r o r s i n radar i n t e r p r e t a t i o n . In 1955 440 MHz pulsed radar a l t i m e t e r s attempted the f i r s t i n t e n t i o n a l v e r t i c a l t r a n s m i s s i o n through i c e on the 800 f t . t h i c k Ross Ice S h e l f . A l a c k of r e s u l t s induced attempts i n 1958 to probe 500 f t . t h i c k i c e at Wilkes S t a t i o n . Strong echoes combined with s e i s m i c i n f o r m a t i o n provided i n f o r m a t i o n about the propagation v e l o c i t y of el e c t r o m a g n e t i c r a d i a t i o n i n i c e . Further work i n Greenland confirmed the d i e l e c t r i c constant value i n i c e of 3.2 (von H i p p e l , 1954). In 1960 t e s t s were made of a l t i m e t e r s o p e r a t i n g at 110, 220, 440 and 4300 MHz. These shewed that the lowest f r e q u e n c i e s were most s u i t e d to sounding of t h i c k i c e . A summary was r e p o r t e d by flaite and Schmidt (1962). In 1963, the B r i t i s h A n t a r c t i c Survey operated the 35 MHz SPRI Mk I apparatus, the f i r s t radar system designed s p e c i f i c a l l y f o r the use of sounding i c e sheets. The SPRI Mk I sounder had a 40 W r.m.s. t r a n s m i t t e r pulse power, a -93 dBm r e c e i v e r s e n s i t i v i t y , a 10 MHz bandwidth and moncpole antennae (Evans, 1963a). Furt h e r t e s t s by the S c o t t P o l a r 2 Sesearch I n s t i t u t e (SPBI) and the U. S. Army E l e c t r o n i c s Laboratory (DSAEL) determined that s i g n a l a t t e n u a t i o n was reduced at lower f r e g u e n c i e s (Evans, 1963b). During the f o l l o w i n g f i e l d season the Ninth S o v i e t A n t a r c t i c E x p e d i t i o n (Bogorodskiy, Rudakov, T y u l ' p i n , 1965) operated a type-1M4 radar apparatus, with a c a r r i e r freguency of 213 MHz, and a pulse power of 80 Kw r.m.s. Strong echoes were obtained from depths up t o 900 m. The p r o j e c t terminated with the l o s s of the apparatus and the d r i v e r i n t o a crevasse. The next t h r e e years saw expanded use of the SPRI Mk II sounder (500 w pulse power, otherwise same as Mk I) and the SCR 718 r a d i o a l t i m e t e r (440 MHz), i n c l u d i n g a i r b o r n e s u r v e y i n g i n Canada, Greenland and A n t a r c t i c a (Evans, 1966). The SPRI operated a SPRI Mk IV sounder from an a i r c r a f t during the 1969-70 f i e l d season. F l i g h t l i n e s are i n Evans and Smith, (1971). The f o l l o w i n g year the SPRI operated a modified SPRI Mk IV apparatus (modified to 60 MHz) from an a i r c r a f t . B u t t e r f l y plan m u l t i - w i r e d i p c l e antennae were used to maintain a l a r g e bandwidth. A maximum i c e t h i c k n e s s cf 4450 m was recorded (Evans, Drewry, Robin, 1972). In 1973 Davis, H a l l i d a y and M i l l e r r e p o r t e d a Cambridge e x p e d i t i o n attempt t o sound the R o s l i n G l e t s c h e r i n Stauning A l p e r , East Greenland, using a modified SCR 718 r a d i o a l t i m e t e r with a 45° corner r e f l e c t o r antenna. Op to t h i s time a l l r a d i o sounding had been with systems eguipped\" with 3 monopole, d i p o l e or two element Yagi antennae. For fr e q u e n c i e s below 300 MHz high gain antennae would be p r o h i b i t i v e l y l a r g e . Sounding i n i c e sheets, however, co u l d be improved by using higher power t r a n s m i t t e r s . Returned power t h a t a r r i v e d l a t e r than the i n i t i a l bottom bounce d i d not a f f e c t the accuracy of the r e c o r d . In v a l l e y g l a c i e r s , wall echoes c o u l d obscure or be mistaken f o r bottom echoes, Davis e t a l (1973) r e c o g n i z e d that i n order to achieve c o n s i s t e n t r e s u l t s r a d i a t e d and r e c e i v e d power must be r e s t r i c t e d t o a narrow beam. Using an antenna with 8 dB forward gain they recorded echoes up to 350 m. In 1972, i n a c o n t i n u i n g p r o j e c t , the Department of Energy Mines and Resources, Canada, made soundings on the Athabasca G l a c i e r , A l b e r t a , Canada, with a UHF apparatus developed by R. Goodman. The u n i t f e a t u r e d a 620 MHz c a r r i e r frequency, 3 Kw pulse power and a very high gain corner r e f l e c t o r antenna. R e s u l t s were g e n e r a l l y negative, but very f i n e i n t e r g l a c i e r s t r u c t u r e was de t e c t e d . The f o l l o w i n g year the u n i t was operated on the T r a p r i d g e and Rusty G l a c i e r s , s u r g i n g g l a c i e r s i n the Yukon T e r r i t o r y , Canada, with t o t a l success (Clarke 6 Goodman, 1975; Goodman et a l , 1975). In 1974, us i n g a t r a n s m i t t e r developed by the S t a n f o r d Research I n s t i t u t e , Watts, Meier et a l (1974) made soundings on the South Cascade G l a c i e r , Washington U.S.A. and the Columbia G l a c i e r , Alaska, U.S.A. The apparatus f e a t u r e d a monopulse s i g n a l with 1-5 MHz c a r r i e r frequency and 100 ns pulse length. Successful soundings were made up to 1200 m in temperate ice (Meier £ Watts, private coffimunicaticn) . Its chie f disadvantage was i n i t s antennae conf igura t ion . I t was necessary to use bu t te r f ly ha l f wave dipoles spread cn the g l ac ie r surface. The transmitter and receiver antennae were separated by 50 m. This conf igurat ion precludes continuous p r o f i l i n g due to i t s s i ze and i t s method of deployment. Radio echo sounding has been establ i shed as a p r a c t i c a l method for obtaining i ce thicknesses , with reasonable r e l i a b i l i t y . Sounding i n large polar i ce masses i s most sa t i s fac tory at low frequencies, the lower l i m i t being determined by antenna s ize and communication inter ference . Large temperate i c e masses could be sounded with low frequency monopole sounders. However sounding in smaller val ley and alpine g l a c i e r s , in p a r t i c u l a r temperate or p a r t i a l l y temperate g l a c i e r s , could be success ful ly studied with UflF sounders. These sounders have very narrow antennae beams, very fast r i set imes and short pulse lengths. They w i l l be the most p r a c t i c a l system for continuous p r o f i l i n g over these g l a c i e r s . 1.2 DIELECTRIC PROPERTIES OF ICE 1.2.1 PERMITTIVITY Pure i c e has a r e l a t i v e l y high s t a t i c p e r m i t t i v i t y (approximately 100) (Evans, 1965) and a long re laxat ion time 5 (0.1 ms) due to i t s polar molecules. At radio frequencies of the order of 1 MHz the e f f e c t s of the relaxation spectrum, and i t s corresponding e f f e c t with temperature cn the r e l a t i v e permittivity have v i r t u a l l y vanished. Numerous programs have detemined that above 1 MHz the r e l a t i v e permittivity cf ice i s 3.2±0.2 (Auty S Cole, 1952; Cumming, 1952; von Hippel, 1954). It i s v i r t u a l l y independent of temperature, and i n temperate ice i t i s suspected that water content i s i n s u f f i c i e n t to t cause large variations (Evans, 1965). 1.2.2 LOSS TANGENT In 0°C ice below 300 MHz, ftanS i s v i r t u a l l y constant, and i n spite of the lack of e f f e c t on p e r m i t t i v i t y , power losses are due primarily to the relaxation spectrum. Above 300 MHz infrared absorption becomes evident. At reduced temperatures, the infrared absorption, which i s r e l a t i v e l y temperature i n s e n s i t i v e , i s evident at lower frequencies. Residual absorption from the relaxation spectrum decreases with lower temperatures (Evans, 1965). In temperate i c e , water content can have .widely varying effects on the loss tangent. This i s due i n part to the wide relaxation spectrum of water, but probably B.C. conductivity of water and scattering by water inclusions provide the greatest losses (Evans, 1965; Smith S Evans, 1972). 6 CHAPTER 2: SYSTEM DESIGN 2.1 GENERAL DESCRIPTION In designing a radio echo sounder for sounding in valley g l a c i e r s one has b a s i c a l l y f i v e parameters to work with: c a r r i e r frequency, bandwidth, pulse length, antenna pattern and peak transmitter power. Echo risetime at the receiver i s determined by transmitter, receiver and antenna bandwidth and by the antenna pattern. Maximum range i s determined by the peak transmitter power, the c a r r i e r freguency, and the antenna pattern. Resolution i s affected by pulse length, system bandwidth and antenna pattern The parameters for a system designed at the University of B r i t i s h Columbia are l i s t e d i n table 2 . 1 . The c a r r i e r frequency i s 840 MHz, the bandwidth i s 40 MHz, the pulse length i s 70 ns, the antenna gain i s 19 dE and the peak r.m.s. transmitter power i s 4.1 Kw. 7 2.2 SELECTION OF THE CARRIER FREQUENCY The d e c i s i o n to operate at 840 MHz can be d i v i d e d i n t o two s t e p s . F i r s t , the d e c i s i o n to operate wi t h i n the U.E.F. T.V. band. Secondly, what frequency w i t h i n the band to use. TRANSMITTER PARAMETERS Operating frequency 840 MHz Bandwidth 35 MHz Pulse l e n g t h 70 ns Rise time 18 ns F a l l time 28 ns Peak power 4.1 Kw (66 dEm) R e p e t i t i o n r a t e 25 KHz ANTENNA CONFIGURATION 90° dual d i p o l e corner r e f l e c t o r Gain 19dB over i s o t r o p i c F r o n t / s i d e r a t i o 60 dB VSWR < 1.5 RECEIVER PARAMETERS Bandwidth 40 MHz Dynamic range 97 dB Minimum s e n s i t i v i t y -82 dEm SYSTEM PERFORMANCE S i n g l e antenna > 95 dE Dual antenna 138 dB TABLE 2.1: RADAR PARAMETERS The prime f a c t o r i n choosing a high frequency i s the c o n s t r a i n t that the f i r s t use of the system w i l l be to sound s m a l l i c e c a p s and v a l l e y g l a c i e r s of the Canadian A r c t i c and Northwest, where v a l l e y w a l l echoes n e c e s s i t a t e a high g a i n antenna. VHF or lower f r e q u e n c i e s cannot be operated with a 8 high gain antenna which would be conveniently small for e i ther a e r i a l or surface use. The useful frequency range i s l imi ted to 300 MHz cr higher (Davis, 1973) . Once the dec i s ion has been made to operate within the useful UHF range (300MHz - 1GHz) a s ingle freguency must be se lected . In sounding - 2 0 ° C i ce at these freguencies f tan 6 increases as the second power of the frequency (Walford, 196 8; Bleaney 8 Bleaney, 1957). This i s countered by the antenna gain, i n that for a given aperture s ize gain also goes up as the second power of the freguency, and the ef fects cance l . The advantage of using a higher freguency l i e s in the narrower beamwidth ava i lab le for a given aperture s i z e . The beam w i l l i l luminate a smaller area of the g l ac i e r bed, hence c learer echoes w i l l be achieved. As the temperature increases the va r i a t ion of ftanS with frequency decreases so that at 0 °C ftan6, or attenuation per unit length i s e f f e c t i v e l y constant (0.057dEm _ 1 : Smith 5 Evans, 1972) hence the advantage of going tc a higher freguency due to having a greater gain for a given aperture i s r e a l . The two way gain increases as the fourth power cf the frequency whereas transmission losses decrease only as the second power. The advantage of the narrower beamwidth s t i l l app l ie s , e spec i a l ly considering that the beam w i l l i l luminate fewer sca t ter ing objects whose range i s close to the range of the bedrock (Davis, 1973), Davis (1973) also suggests that 9 s c a t t e r e d power may a c t u a l l y tend to decrease with these higher f r e q u e n c i e s , although evidence f o r t h i s i s l i m i t e d . The f i n a l d e c i s i o n to use 840 MHz r a t h e r than any nearby frequency ( e i t h e r higher or lower) i s economic. 840 MHz ± 20MHz l i e s w i t h i n a common c a r r i e r landbased mobile, and i n t e r i m UHF TV band f o r which hardware i s more e a s i l y a v a i l a b l e . Due t o i t s dual use, t h i s frequency has been set as a break p o i n t i n t r a n s m i t t e r d e s i g n ; any lower frequency would r e q u i r e a l a r g e r , more expensive c a v i t y a m p l i f i e r (R.L. Sepulveda, Microwave C o n t r o l Co,, p r i v a t e communication) . 2.2.1 TRANSMITTER DESIGN At the c a r r i e r frequency s e l e c t e d i t i s more s t a b l e to employ a c r y s t a l frequency generator r a t h e r than use an L-C tank to produce the frequency, as i s the case with more c o n v e n t i o n a l systems such as the SPRI Mk I I . The t r a n s m i t t e r d e r i v e s i t s frequency from a 120 MHz c r y s t a l o s c i l l a t o r , fed i n t o a x7 m u l t i p l i e r . Two stages of a m p l i f i c a t i o n and modulation l e a d the s i g n a l to a c a v i t y t r i o d e R.F. power a m p l i f i e r . Here some of the advantages of the c r y s t a l generated frequency become apparent. It i s much e a s i e r to detune the c h a r a c t e r i s t i c l y high Q of a c a v i t y a m p l i f i e r , g i v i n g the e f f i c i e n t r e s u l t of having a l l of the t r a n s m i t t e d power w i t h i n the r e c e i v e r bandwidth. 10 2.2.2 ANTENNA S TRANSKIT/RECEIVE SWITCH A corner r e f l e c t o r antenna was selected for the prototype conf igura t ion . With a two co l inear dipole driven element the antenna should have s u f f i c i e n t l y high gain and narrow beam width to y i e l d strong echoes with short fading patterns . Concurrently the 9 0 ° s ide lobes should be s u f f i c i e n t l y low that va l ley wall echoes w i l l not obscure bottom echoes when the apparatus i s operated at the ice surface. A c i r c u l a t o r i s used as a passive transmit /receive switch when the system i s operated with a s ingle antenna. 2.2.3 RECEIVER AND RECORDER The rece iver has a logari thmic intermediate freguency (I.F.) ampl i f ier as i t s primary component. I n i t i a l l y a photographic X-Y or intensity-modulated recording w i l l be used. In the future i t i s hoped to be able to record the s ignals d i g i t a l l y onto magnetic tapes. 2.3 SYSTEM PERFORMANCE 5 RANGE The smaller more e f f i c i e n t c a v i t i e s ava i l ab le at th i s freguency y i e l d higher peak power, hence greater system performance. Important parameters are l i s t e d in Table 2 .2 . If we assume d i e l e c t r i c losses B of 0.057dEm _ 1 and a bedrock r e f l e c t i o n c o e f f i c i e n t R of -20 dB (Rcbin, Evans, Ba i l ey , 1969; Harr i son , 1972) an antenna gain G of 19 dE and a 11 system performance of 138 dB then the maximum range i s determined by p , received _ G^X^R .. -0.2Dr T~T 10 (1) P. , 64iTr transmitted Peak power 66 dBm Receiver s e n s i t i v i t y --90 dEm Mixer c o n v e r s i o n l o s s -8 dB Maximum system performance 138 dB L i n e a r system performance 118 dB 1 1 R e c e i v e r i s l i n e a r down to -70 dBm. TABLE 2.2: SYSTEM PERFORMANCE The maximum range r i s then 700 m which should be ample f o r sounding many v a l l e y g l a c i e r s . . According to Davis (1973) a c r i t e r i o n f o r s u c c e s s f u l sounding of temperate g l a c i e r s i s p r ( p l a n £ ) = f ( f ^ ) >1 (2) r(scattered) m where R i s the bedrock r e f l e c t i v i t y , assumed t o be -20 dE; C i s the s c a t t e r i n g c o e f f i c i e n t , c a l c u l a t e d to be 0.01 m~l (based on Davis* (1973) a n a l y s i s of the DEMB sounder o p e r a t i n g at 620 MHz); G i s the antenna g a i n , set at 19 dE and 1 i s the pulse l e n g t h i n i c e . In our case R.G-l 0.01 80-1 CK 1 ; 0.01/m^ l W ' ( 3 ) m 12 hence our system should be s u c c e s s f u l at sounding temperate g l a c i e r s . 2 .4 ANTENNA DESIGN There are numerous s t y l e s of antennae which can provide l a r g e bandwidth and high g a i n a t the f r e g u e n c i e s d i s c u s s e d here. Some are capable of very great brcadbanding. The Yagi, p a r a b o l o i d , h e l i c a l and broadside a r r a y are some. However, i n s e l e c t i n g an antenna t o sound v a l l e y g l a c i e r s , forward gain i s not the o n l y c o n s t r a i n t . I g u a l l y important i s the 90° s i d e lobe l e v e l , or the \" f r o n t to s i d e \" r a t i o . In a v a l l e y , the d i p of the v a l l e y w a l l s r a r e l y exceeds 4 5 0 [ F i g * 2.1]. I f we model a g l a c i e r as f i l l i n g approximately a p a r a b o l c i d a l v a l l e y i t f o l l o w s t h a t over a l a r g e range of d i s t a n c e s from the v a l l e y w a l l , L, the t r a v e l time f o r the t r a n s m i t t e d pulse to reach the v a l l e y w a l l c l o s e l y matches the time f o r t h a t pulse to reach the bed, a t height H below the i c e s u r f a c e (using a v e l o c i t y i n i c e c f 176 m/ s e c ) . S i n c e the s u r f a c e wave i s not n o t i c a b l y a t t e n u a t e d , i n a l a r g e g l a c i e r the v a l l e y w a l l echo may e a s i l y obscure or be confused with the bottom echo. For example the p a r a b o l o i d a l r e f l e c t o r antenna has a f r o n t to s i d e r a t i o of 30 dB. With 0.05 dBm - 1 a t t e n u a t i o n i n i c e the bottom echo w i l l be the same s t r e n g t h as the w a l l echo Valley wall d i p U s g -a H FIGURE 2.1 14 a f t e r only 600 m of two way t r a v e l . In temperate i c e t h i s maximum range decreases r a p i d l y . I d e a l l y the f r o n t to s i d e r a t i o should not be g r e a t e r than one h a l f the system performance i n order tc have pcwer returned from the w a l l s obscured i n system n o i s e . It i s p o s s i b l e to achieve a f r o n t t o s i d e r a t i o of 60 dB by using a corner r e f l e c t o r . T h i s w i l l reduce system performance to 120 dB when operated i n such a v a l l e y . aperture s y n t h e s i s may improve the f r o n t t o s i d e r a t i o s t i l l f u r t h e r , though t h i s technique i s d i f f i c u l t and n e c e s s a r i l y very slew. 15 CHAPTER 2k CIRCUIT DESIGH The radio echo sounder consis t s of f ive units (not including power supply) connected i n the f i e l d only during use. They are the t ransmit ter , c i r c u l a t o r , antenna, rece iver /pulse generator, and d i sp lay . The assembly i s depicted i n Figure 3 .1 . 3.1 TRANSMITTER CIRCUIT The transmitter was b u i l t for U . E . C . by Microwave Control Company, Farmingdale, New Jersey, U .S .A . A block diagram for the transmitter i s shown i n Figure 3.2. The c a r r i e r freguency i s generated by a c r y s t a l o s c i l l a t o r operating at a 120 MHz harmonic. The s igna l i s then ampli f ied by two NPN t r a n s i s t o r s , both operating c la s s A (2N3866). Two more stages of ampl i f i ca t ion buffer the 120 MHz c a r r i e r . The 120 MHz s igna l i s fed to p a r a l l e l x6 and x7 m u l t i p l i e r s which y i e l d 720 MHz s ignals for the l o c a l o s c i l l a t o r , and 840 MHz for the broadcast c a r r i e r . The 720 MHz s igna l passes through a bandpass f i l t e r to the l o c a l o s c i l l a t o r output jack. The 840 MHz c a r r i e r then drives two stages of tuned common base ampl i f i e r s . The f i r s t stage i s gated by the modulator pulse which has been amplif ied from a TTL pulse provided by the external pulse generator. The second stage provides a maximum B. F. power of 20 watts. CIRCULATOR TRANSMITTER SMA SMA LOCAL OSCILLATOR TRIGGER PULSE SMA SMA RECEIVER N N BNC VIDEO BNC ANTENNA BNC DISPLAY TRIGGER BNC VERTICAL SCAN BNC DISPLAY BNC FIGURE 3.1: RADAR INTERCONNECTION DIAGRAM v V W V S M SOLID STATE AMPLIFIER TRIODE AMPLIFIER TIMES 7 MULTIPLIER LOCAL OSCILLATOR OUT PULSE AMPLIFIER TIMES 6 120 MHz AMPLIFIER MULTIPLIER / 720 MHz ' BANDPASS 120 MHz OSCILLATOR TRIODE AMPLIFIER R.F. OUT ISOLATOR SMA TRIGGER IN POWER SUPPLY / 26v REGULATOR e -o 30v IN FIGURE 3.2 RADAR TRANSMITTER BLOCK DIAGRAM h-1 18 The 20 watt R. F. power then passes through a 30 watt i s o l a t o r to a t r i o d e a m p l i f i e r / m o d u l a t c r , which i s a l s o gated by an a m p l i f i e d pulse from the e x t e r n a l pulse generator. The high power modulated R. F, s i g n a l i s then fed f i n a l l y t c a broad band t r i o d e power a m p l i f i e r which d e l i v e r s 4.1 k i l o w a t t s peak R. F. power through an i s o l a t o r to the R. F. output jack. i Power f o r the t r a n s m i t t e r i s drawn from a 28 v - 30 v supply which i s dropped and r e g u l a t e d to 26 v. A l l v o l t a g e s used (up to 4 Kv) are d e r i v e d from t h i s 26 v r e g u l a t e d supply. A d i g i t a l timing c i r c u i t d i s a b l e s the high v o l t a g e s f o r 1.5 minutes a f t e r turnon to allow f o r warmup. I n t e r n a l sensing c i r c u i t s provide f o r immediate shutdown should any s u b c i r c u i t o v e r l o a d , f o r any reason. The shutdown c i r c u i t throws a c i r c u i t breaker which may be r e s e t by a p p l y i n g a p o t e n t i a l t o a p i n on a t e s t jack provided. 3.2 RECEIVER/PULSE GENERATOR CIRCUIT The r e c e i v e r / p u l s e generator was designed and assembled at the U n i v e r s i t y of B r i t i s h Columbia, Department of Geophysics. F i g u r e 3.3 i s a block diagram f o r the r e c e i v e r / p u l s e generator. The r e c e i v e d R. F. s i g n a l passes through a diode switch which i s enabled during the t r a n s m i t pulse when the system i s DIODE SWITCH MIXER R.F. IN LOG I.F. AMPLIFIER VIDEO AMPLIFIER ATTENUATOR SMA LOCAL OSCILLATOR IN PULSE GENERATOR 100 KHz MULTIVIBRATOR SYNCRONOUS DIVIDE BY 4 A i DIODE SWITCH PULSE FORMER TRANSMITTER! TRIG. DELAY TRANSMIT PULSE FORMER FIGURE 3.3 6 C RADAR RECEIVER BLOCK DIAGRAM D 6 BNC VIDEO OUT TRANSMIT TRIGGER 3 \" SMA BNC -4 DISPLAY TRIGGER i-1 20 operated i n the single antenna mode. The diode switch i s enabled by an amplified pulse derived from the pulse generator. The R. F. s i g n a l i s then mixed down tc the 120 MHz intermediate freguency, using the 720 MHz l o c a l o s c i l l a t o r provided i n the transmitter. The l o c a l o s c i l l a t o r provides 22 dBm of R. F. power to the mixer. This i s attenuated to 10 dBm, The mixer i s a Mini C i r c u i t s Laboratory model MA-1 with a conversion loss of 8 dB, The I, F. s i g n a l i s then amplified through a logarithmic I. F, amplifier. The amplifier i s an EHG model ICLT12040. It has a center frequency at 120 MHz, a 3 dB bandwidth of 40,1 MHz. I t has a dynamic range of 97 dE and i s l i n e a r l y logarithmic (±1 dB) for 70 dB of i t s range. Its risetime i s less than 20 ns; i t s noise figure i s 10 dE. Its output voltage range i s from 0,017 v at -90 dBm to 2,670 v at +7 dBm. The detected video signal passes through one f i n a l stage of gain which adjusts amplitude and bias for display purposes. The pulse generator provides trigger pulses to the transmitter, diode switch and oscilloscope display. I t i s capable of only a single r e p e t i t i o n rate and pulse length. The r e p e t i t i o n rate i s determined by a 100 KHz TTL multivibrator. The 100 KHz clock pulse then passes through a divide-by-four TTL counter which provides the 25 KEz trigger pulse rate. The counter provides two b i t s cf control information for a proposed d i g i t i z e r . The 25 KHz pulse then 21 drives two monostable TTL f l i p f l o p s . One monostable pulse i s amplif ied and enables the diode switch. The second mcnostable pulse provides a time delay between the diode switch enable pulse and a t h i r d monostable TTL f l i p f l o p . The t h i r d monostable v ibra tor i s adjusted to have a pulse length of 70ns. This pulse provides the modulating s igna l to the transmitter and the t r igger for the osc i l loscope d i sp l ay . The length of the diode switch enable pulse i s adjusted so that the switch and the transmitter disable simultaneously. 3.3 DISPLAY AND RECORDER The i n i t i a l display o sc i l lo scope i s a Tektronics model 475 o s c i l l i s c o p e with a Polaro id f i lm pack. When operated using i n t e n s i t y modulation a ramp generator scans the v e r t i c a l axis on the o sc i l l o s cope , which exposes one frame. The ramp generator cons i s t s of a var iable clock pulse which drives a nine b i t counter, which drives a d i g i t a l to analog converter . Intensity and modulation amplitude are contro l l ed by the video ampl i f i e r . A block diagram appears in Figure 3.4. In addi t ion i t i s hoped eventual ly to be able to d i g i t i z e and record the complete radar record . The proposed d i g i t i z e r w i l l be capable of 1024 channels with 10 ns channel separat ion.the d i g i t i z e r w i l l sample 128 pulses at one time 22 delay and average them. The time delay w i l l then be incremented by 10 ns and the process w i l l c o ntinue. The analog c i r c u i t r y i s based on a sample and hold a m p l i f i e r with 175 MHz bandwidth designed by H. B a l d i s and J . Aa2am-Zangeneih. The output w i l l be e i g h t b i t p a r a l l e l . A block diagram appears i n F i g u r e 3.5. + 6v O EXAR 2240M c l o c k s t a r t e i g h t b i t counter J i stop -4A/V -AAAr v e r t i c a l scan FIGURE 3.4: INTENSITY MODULATION RECORDING BLOCK DIAGRAM ( v e r t i c a l scan) N J C O DATA OUT OUTPUT FLAG data OUTPUT BUFFER data 15 BIT REGISTER clock clear A o 100 KHz IN clock] 15 BIT ADDER X data 8 BIT A/D CONVERTER X 1 BNC / VIDEO IN T 10 ns SAMPLE & HOLD ENABLE DECODER B C 10 BIT SAMPLE & HOLD ZERO DETECT TRIGGER 100 MHz OSCILLATOR 10 BIT ECL TIME DELAY COUNTER T 10 BIT ECL CHANNEL SELECT COUNTER -O D 7 BIT. COUNTER 1 FIGURE 3.5 RADAR DIGITIZER BLOCK DIAGRAM 4>-25 CHAPTER 4: SYSTEM CAPABILITIES 4.1 SYSTEM PERFORMANCE The maximum range of any radar system i s a f u n c t i o n of four parameters: antenna g a i n , frequency, medium a t t e n u a t i o n and system performance. The l a t t e r i s de f i n e d as the maximum r.m.s. t r a n s m i t t e d power d i v i d e d by the r e c e i v e r s e n s i t i v i t y , thus system performance i s a f u n c t i o n of the t r a n s m i t t e r and r e c e i v e r parameters, but i t i s a l s o p o s s i b l y determined by antenna c h a r a c t e r i s t i c s . I t i s p o s s i b l e t h a t there i s s u f f i c i e n t t r a n s m i t t e r leakage power, which may be a v a i l a b l e t o the r e c e i v e r , and could obscure otherwise usable echoes. In the present case t h i s i s a r e a l c o n s i d e r a t i o n when a s i n g l e antenna i s used. I f the ON/OFF t r a n s m i t t e r r a t i o i s only 100 dE and the c i r c u l a t o r i s o l a t i o n i s 2 5 dB, then system performance c o u l d be l i m i t e d to t h e i r sum, 1 2 5 dB. F u r t h e r , i f the antenna i s mismatched so t h a t i t has a r e f l e c t i o n c o e f f i c i e n t g r e a t e r than - 2 5 dB, then the system performance would be degraded s t i l l f u r t h e r . Any attempt to improve system performance by improving r e c e i v e r s e n s i t i v i t y would be wasted. T h i s problem can be avoided by employing two antennae as with the SPRI Mk I I sounder (Evans 8 Smith, 1 9 6 9 ) . It i s r e l a t i v e l y easy t o produce antennae p a i r s with the r e q u i r e d 26 i s o l a t i o n , p a r t i c u l a r l y i f high gain antennae are used. Another, poss ibly less expensive method i s ava i lable for increas ing i s o l a t i o n . Passive or act ive diode switches operated between the transmitter and c i r c u l a t o r can increase the ON/OFF r a t i o by at least 40 dB. The only reguirement cn the switch i s the a b i l i t y to conduct the maximum transmitter power, with minimum lo s s . Switching speed i s net c r i t i c a l as the switch need only be off for maximum range echoes, usual ly several microseconds after the transmitter pulse. 4.2 MAXIMUM RANGE AND RESOLUTION In radio echo sounding i n i ce the bedrock has general ly been modeled as a specular plane r e f l e c t o r with a r e f l e c t i o n coe f f i c i en t of -20 dB. This f igure has been considered conservative (Davis, 1973). Deviations on t h i s model have also been considered (Harrison, 1974). I f the plane model i s assumed then the maximum range i s determined by P O ; received G ^ R . -0.2Dr ... F , \" 6 4 ^ 1 0 ( 1 ) transmitted where the l e f t expression eguals the inverse system performance, G i s the free space antenna ga in , A i s the free space wavelength, R i s the bedrock r e f l e c t i v i t y , D i s the loss in dBm - 1 , and r i s the range. In cold i c e , i f the attenuation of the s ignal i s 0.057 d B m - 1 , then the maximum range i s 700 m, with the 27 des c r i b e d apparatus, as st a t e d i n Chapter 2. In temperate i c e the maximum range i s only 210 tn f o r a t o t a l a t t e n u a t i o n of 0.2 dBm-*. I f t o t a l a t t e n u a t i o n i s 0.15 dBm-1 (Ragle e t a l , 1964), then the maximum range i s 280 m. Improved gain due to r e f r a c t i o n at the i c e s u r f a c e may improve the system performance by about 2 dB i n c r e a s i n g the maximum range to 290 m i n the l a s t case. I f p r e c i s i o n i s d e f i n e d as the time f o r echo power to r i s e 3 dB above l o c a l average power, then i t f o l l o w s that p r e c i s i o n i s a constant f r a c t i o n of range. In the present case, p r e c i s i o n i s 18ns-88m/ys _ zoUm of range i n temperate i c e . In c o l d i c e the p r e c i s i o n i s 0.25% of range. (Note: these f i g u r e s are based upon the assumption t h a t the plane r e f l e c t o r i s s u f f i c i e n t l y s p e c u l a r , t h a t f o r the high gain antenna, the plane i s e f f e c t i v e l y i d e n t i c a l to a s p h e r i c a l r e f l e c t o r centered a t the antenna with r a d i u s equal to the r e a l range. The v a l i d i t y of the assumption i s s o l e l y a f u n c t i o n of the antenna gain and beamwidth. If fadin g p a t t e r n s r e s u l t i n g from too broad a beam degrade the echo r i s e t i m e , a d e c o n v o l u t i o n of the echo, i f p o s s i b l e , using the t r a n s m i t t e d p u l s e as a source f u n c t i o n , w i l l r e t u r n t h i s a c c u r a c y ) . 28 4.3 REFLECTIVITY OF LAYERS WITHIN A GLACIER Ha r r i s o n (1973) has shown t h a t the r e f l e c t i o n c o e f f i c i e n t R, f o r random v a r i a t i o n s i n p e r m i t t i v i t y vary with the pulse length i n r a d i a n s L, as R - i ^ r ) L 2 e \" 2 L i (5) where l a y e r t h i c k n e s s i s g r e a t e r than the pulse l e n g t h . ^E' i s a step change of the r e l a t i v e p e r m i t t i v i t y e with depth. H a r r i s o n has concluded that i t i s necessary to r e s t r i c t o n e s e l f to the case where the l a y e r t h i c k n e s s i s much l e s s than the pulse l e n g t h . In t h i s case T T 2 L 2 . 2 R - -ft {^fr} (6) m f o r s i n g l e r e f l e c t o r s , and 7 T 3 p L . 2 - k 2 L 2 _ _ m m fAS i e m m . f o r m u l t i p l e r e f l e c t o r s where 1^ i s the l a y e r t h i c k n e s s or s p a c i n g , ^ m i s the wavelength i n i c e , P m i s the pulse length i n i c e and k m i s the wave number i n i c e . H a r r i s o n has a l s o shown that there i s a maximum r e f l e c t i v i t y at l m = 1/^2 where •m A 2 -h & m a v = T7\" P m k {—^} (8) max lb mm e In the present case, t h i s reduces to A F 2 R = 0.119 {-^ T} (9) max e 29 at L =2.5 cm. To obtain a power r e f l e c t i o n m -70 dB in t h i s case would require that Ae = 10~3 e It i s clear that UHF radio echo sounders can be to small variations in e over centimeter ranges. 4.4 POLARIZATION 4.4.1 OPTICAL ACTIVITY Optical a c t i v i t y i s usually described as the tendency of transparent matter to rotate the E-vector of plane polarized electromagnetic radiation. The E-vectcr of any radiation traversing o p t i c a l l y active matter w i l l rotate either clockwise or counterclockwise when viewed from the radiation source. With a radio echo sounder, plane polarized radiation enters a g l a c i e r at normal incidence. Any rotation due to op t i c a l a c t i v i t y i n the i c e during the f i r s t t r a n s i t (to the bottom) should equal the rotation due to the return traverse. Since the rotation sense does not change, the t o t a l rotation should cancel. Hence radio echo sounding cannot detect o p t i c a l a c t i v i t y . c o e f f i c i e n t of (10) very sensitive 4.4.2 DOUBLE REFRACTION Double r e f r a c t i o n (also known as birefringence) i s 30 defined as a difference i n r e f r a c t i v e index for radiation with E-vectors normal and p a r a l l e l to a well defined c r y s t a l axis (Jenkins & White, 1957). At v i s i b l e wavelengths the ordinary and extraordinary indices of r e f r a c t i o n in ice d i f f e r by 0.3%. In the i d e a l s i t u a t i o n , with c r y s t a l s having their o p t i c a l axes p a r a l l e l and horizontal, a 90° phase s h i f t of the ordinary and extraordinary rays at 840 HHz could occur i n only 16 meters of two way t r a n s i t ( i t i s necessary to assume that the r a t i o of r e f r a c t i v e indices i s constant with frequency above the relaxation spectrum). However t h i s i s excessively optimistic for two reasons. F i r s t , there i s a tendency for the o p t i c a l axes of i c e c r y s t a l s i n glaciers to a l i g n v e r t i c a l l y (Paterson, 1969). Secondly, and predominantly, nonuniforrcity in axis orientation causes double r e f r a c t i o n to cancel. Consider an orthonormal base vector set, say v e r t i c a l , in the d i r e c t i o n of glacier flow, and horizontal and normal to glacier flow. By d i r e c t i o n cosines the g l a c i e r can be divided into three equivalent thicknesses of ice with pure c r y s t a l axis orientations. The component with v e r t i c a l axes does not exhibit double r e f r a c t i o n and can be ignored. The equivalent thickness of ice available for a f f e c t i n g double re f r a c t i o n i s equal to the difference of the other two components. Hence i t i s necessary to have a very strong prefered horizontal orientation to make double r e f r a c t i o n v i s i b l e . Sounding at frequencies of about 60 HHz, double refraction would require 31 at least 120 m of ice to produce a 90° phase sh i f t . Jiracek (1965) reported that at South Pole Station, using a 30 MHz system, a bottom echo could not be received K i t h antennae broadside and para l le l . Maximum receiver power occurred when the antennae were perpendicular and horizontal. Jiracek interpreted this as a 9 0 ° rotation from the original transmitted pulse. \"On the Skelton Glacier , bottom echo amplitude was practically independent of receiving antenna orientation in the horizontal plane.\" Jiracek interpreted this as a transformation of l iner polarized radiation into near circular polarization by double refraction. 4.4.3 DETECTING DC0BL1 REFRACTION The orthonormal basis with one axis ver t ica l , which maximizes the effective double refracting thickness defines the privileged directions in i t s two horizontal axes. These are the directions in which a l l of the transmitted power is either in the ordinary ray or the extraordinary ray. In these directions double refraction i s not v i s ib le . Bowever upon arr ival at the bedrock the ideal reflector model f a i l s by returning only a component fraction of polarized power instead of maintaining a l l polarization. The remaining reflected power i s returned unpolarized. A receiver antenna based cn dipoles, when rotated cannot distinguish between e l i p t i c a l l y polarized radiation and combined linear and unpolarized 32 radiation. A l l that i s possible i s a determination of the major axis of returned power. If t h i s d i f f e r s from the transmitted axis, then double r e f r a c t i o n has been detected. By rotating the transmitter antenna and repeating the procedure two transmitter antenna orientations should appear where no double r e f r a c t i o n i s evident, i . e . the major receiver power axis coincides with the transmitter pcwer axis. At these positions an estimate for bedrock depolarization can be made. Once t h i s i s known i t may be possible tc estimate the amount of double re f r a c t i o n and hence draw a conclusion about the ice f a b r i c . 4.4.4 FARADAI EFFECT In many substances when plane-polarized radiation traverses i n a d i r e c t i o n p a r a l l e l to an applied magnetic f i e l d , the plane of vibration i s rotated. The amount of rotation Qis proportional to the f i e l d strength fi, and tc the distance traversed L. i . e . 0 = VLH where V, the Verdet constant i s determined by the substance and the wavelength. For water M equals 0.0131 minutes Oersted - 1cm - 1 at the Sodium D l i n e s . In the i n f r a r e d V i s the order of 10~ 3 minutes Oersted _ lcm-* (Jiracek, 1967). When radiation i s r e f l e c t e d back through the medium the f i e l d d i r e c t i o n i s e f f e c t i v e l y reversed, and the rotation due 33 to the second t r a v e r s e adds to the r o t a t i o n due tc the f i r s t t r a v e r s e . T h i s i s u n l i k e n a t u r a l o p t i c a l a c t i v i t y which c a n c e l s e x a c t l y . I f Faraday r o t a t i o n i n i c e due to the l c c a l magnetic f i e l d i s d e t e c t a b l e , i t can be d i s t i n g u i s h e d from double r e f r a c t i o n by o b serving the average angular v a r i a t i o n of r e c e i v e r maximum from t r a n s m i t t e r o r i e n t a t i o n , averaged over a l l t r a n s m i t t e r d i r e c t i o n s . J i r a c e k (1965) has c o n s i d e r e d the magnitude of the Faraday e f f e c t at the South P o l e . He determined that r o t a t i o n i n the i n f r a - r e d would be only 5 ° . Since the Verdet constant decreases with frequency ( J i r a c e k , 1965) the e f f e c t should be n e g l i g i b l y s m a l l , even i n the very t h i c k e s t i c e . 4.4.5 PHOTO-ELASTICITY A t r a n s p a r e n t i s o t r o p i c medium becomes o p t i c a l l y a n i s o t r o p i c when s u b j e c t e d to mechanical s t r e s s . The p r i v i l e g e d d i r e c t i o n s are along the d i r e c t i o n s of the p r i n c i p a l s t r e s s e s . Since p h o t o - e l a s t i c i t y i s o p t i c a l l y i d e n t i c a l to double r e f r a c t i o n , the two e f f e c t s cannot be d i s t i n g u i s h e d . I t i s u n l i k e l y that any i n f o r m a t i o n r e g a r d i n g the i c e f a b r i c can be determined a p o s t e r i o r i from i n f o r m a t i o n gathered by s t u d y i n g the e f f e c t s of a l p i n e or s m a l l v a l l e y g l a c i e r s upon p o l a r i z e d r a d i o echo sounder r a d i a t i o n . A l l that may be p o s s i b l e i s the d e t e c t i o n of one or more of these 34 e f f e c t s , 4.4.6 DIGITAL RECORDS A proposed d i g i t i z e r having 8 b i t r e s o l u t i o n and 10 ns channel s e p a r a t i o n w i l l be used to r e c o r d the r e c e i v e d envelope as provided by the output of the video a m p l i f i e r . 10 ns channel s e p a r a t i o n y i e l d s a Nyguist frequency of 50 HHz so t h a t no r e c e i v e r power w i l l a l i a s i n t o a lower frequency. By deconvolving f a d i n g p a t t e r n s using the t r a n s m i t t e r pulse as a source f u n c t i o n , and averaging over s e v e r a l records gr e a t e r rangy accuracy should r e s u l t and i n t e r m e d i a t e r e f l e c t o r s i n the i c e should become v i s i b l e . Gross bottom roughness may be estimated by observing the spread i n time of r e t u r n e d power from the bottom echo. Of p a r t i c u l a r i n t e r e s t would be the d e t e c t i o n of f i n e i n t e r g l a c i e r s t r u c t u r e near the bedrock. 4.5 CONCLUSION T h i s t h e s i s has presented a b a s i s f o r the design of a UHF r a d i o echo sounder f o r the purpose of s t u d y i n g g l a c i e r s i n mountainous t e r r a i n , S p e c i f i c design parameters have been presented f o r a r a d i o echo sounder to be b u i l t at the U n i v e r s i t y of B r i t i s h Columbia. These parameters have been discus s e d and a c c o r d i n g to normal r a d i o a l t i m e t r y p r a c t i c e s and e n g i n e e r i n g p r i n c i p l e s p r a c t i c a l l i m i t s cf the technique 35 have been proposed. The design has f o l l o w e d as a b a s i s a r a d i o echo sounder b u i l t and operated by the Department of Energy, Mines and Resources, Ottawa, Canada, and Environment Canada, o p e r a t i n g at 620 MHz. He suggest that UHF r a d i o echo sounding w i l l provide a h i g h l y mobile method of sounding the a l p i n e and v a l l e y g l a c i e r s which have p r e v i o u s l y evaded s u c c e s s f u l study. 36 BIBLIOGRAPHY - Auty, R. P., and C o l e , R. H. 1952. D i e l e c t r i c p r o p e r t i e s of i c e and s o l i d D20. The J o u r n a l of Chemical P h j s i c s , V o l . 20, No, 8, p. 1309-1314. B a i l e y , J. T., Evans, S, and Robin, G. de Q. 1964. Radio echo sounding of p o l a r i c e s h e e t s . Nature, V o l . 204, No. 4967, p. 420-421. B a l d i s , H. A,, and Aazam-Zanganeh, J , 1973, High speed s i n g l e event sampler. Reviews of S c i e n t i f i c Instruments, V o l . 44, No. 6, p. 712-714. Beckmann, P., and S p i z z i c h i n c , A. 1963. The s c a t t e r i n g of e l e c t r o m a g n e t i c waves from rough s u r f a c e s . Oxford^ Pergamon Press. Berry, M. 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E l e c t r o m a g n e t i c r e f l e c t i o n from m u l t i l a y e r e d models. P£2£Si^iH3s of the Symposium cn Remote Sensing i n G l a c i o l o a y ~ C a m b r i d g e x September J974, i n p r e s s . ~~ ~ Longhurst, R. S, 1957, Geometrical and p h y s i c a l o p t i c s . London^ Longmans^ Green and Co. Nye, J . F., Kyte, R. G., and T h r e l f a l l , D. C. 1972. Proposal f o r measuring the movement c f a l a r g e i c e sheet by observing r a d i o echos. J o u r n a l of Glaciolocjy, V o l . 11, No. 63, p. 319-325. Oswald, G. K. A. 1974. I n v e s t i g a t i o n of s u b - i c e bedrock c h a r a c t e r i s t i c s . Proceedings of the Symposium on Remote Sensing i n G l a c i o l o g y x Cambridqe x September .1974, i n press. Page, D. F., and Ramseier, R, 0. 1974. A p p l i c a t i o n c f a c t i v e radar techniques to the study cf i c e and snow. 2^ the Symposium on Remote Sensing i n SiSSiSlSSIjt Cambridge x September 1974, i n p r e s s . Paren, J. G. 1970, D i e l e c t r i c p r o p e r t i e s of i c e , Ph.D. t h e s i s , Darwin C o l l e g e , Cambridge. Paterson, W. S. B. 1969, The P h y s i c s of G l a c i e r s . O x f o r d x Pergamon P r e s s . 42 Binker, J . N. 1964. Radio echo sounding and s t ra in rate measurement i n the ice sheet of North-west Greenland; 1964. Polar Record, Vo l . 12, No. 79, p. 403-405. Ragle, R. H . , and others . 1964. Ice cere studies cf Ward Hunt Ice Shel f , by R. H. Ragle, R. G. B l a i r and L. E. Persson. Journal of g lac io loc j j . V o l . 5, No. 37, p. 39-59. Robin, G. de Q , , Evans, S . , Drewry, C. J . , Harr ison, C. E . , and Pe t r i e , D. L. 1970. Radio echo sounding cf the Antarc t i c Ice Sheet. Antarc t ic Journal , V o l . 6, p. 229-232. Robin, G. de Q. , Swithinbank, C. W. M . , and Smith, B. M. E. 1968. Radio echo explorat ion of the Antarc t ic Ice Sheet. IASH Publ ica t ion 86, p. 97-115. Ross i ter , J . R., and others , 1973. Radio interferometry depth sounding: part II - experimental r e s u l t s , by J . R. Ros s i t e r , G. A. LaTorraca, A. A. Annan, C. W, Strangway and G. Simmons. Geophysics, V o l . 38, No. 3, p. 581-599. Smith, B. H, E. 1971. Radio echo studies of g l a c i e r s . Ph.D. thes i s , Cambridge U n i v e r s i t y . Smith B. M. E . , and Evans, S. 1972. Radio echo sounding: absorption and scat ter ing by water inc lus ions and ice lenses . Journal of G lac io loay , V o l . 11, No. 61, p. 133-146. Strangway, D. W., Simmons, G . , LaTorraca, G . , Watts, R . , Bannister , L . , Baker, R . , Redman, J. C . , and Ross i ter , J . R. 1974. Radio-frequency interferometry - a new technigue for studying g l a c i e r s . Journal of Glacioloc], V o l . 13, No. 67, p. 123-132. 43 Swithinbank, C. 1968. Radio echo sounding cf A n t a r c t i c g l a c i e r s from l i g h t a i r c r a f t . IASH P u b l i c a t i o n 79, p. 405-414. Swithinbank, C. W. M. 1972. F i e l d work. Radio Echo Sounding by the B r i t i s h A n t a r c t i c Survey. Polar Record, Vo l . 16, No. 102, p. 411-412. Von H i p p e l , A. 1954. D i e l e c t r i c m a t e r i a l s and a p p l i c a t i o n s . New Y o r k x Technology Press and Wiley, p. 12, p. 301. Waite, A. H. J r . 1966. I n t e r n a t i o n a l experiments i n g l a c i e r sounding, 196 3 and 1964. Canadian J o u r n a l of E a r t h S c i e n c e s , V o l . 3, No. 6, paper 17, p. 887-892. Waite, A. H. , and Schmidt, S. J . 1962. Gross e r r o r s i n height i n d i c a t i o n from pulsed radar a l t i m e t e r s o p e r a t i o n over t h i c k i c e or snow. Proceedings cf the I n s t i t i u t e of Radio E n g i n e e r s , V o l . 50,~No.~ 6~ p. 1515-1520. Walford, M. E. R. 1964. Radio echo sounding through an i c e s h e l f . Nature.* V o l . 204, No. 4956, p. 317-319. Walford, M. E. R. 1965. Radio echo sounding cf p o l a r i c e masses. Ph.D. t h e s i s , Cambridge U n i v e r s i t y . Walford, M. E. R. 1968. F i e l d measurements of d i e l e c t r i c a b s o r p t i o n i n A n t a r c t i c i c e and snow at very high f r e g u e n c i e s . J o u r n a l of G l a c i o l o g y , V o l . 7, No. 49, p. 89-94. . Walford, M. E. R. 1972. G l a c i e r movement measured with a r a d i o echo t e c h n i q u e . Nature, V o l . 239, p. 93-95. Watts, B. D., England, A. W., Meier, M. F., and V i c k e r s , R. S. 1974. Radio echo sounding of temperate g l a c i e r s at f r e g u e n c i e s of 1 to 5 MHz. Proceedings of the Symposium on Remote Sensing i n G l a c i o l o g y ^ Cambridge^ September J974, i n press. 44 Weber, J . R., and Andrieux, P. 1970. Radar sounding of the Penny Icecap, B a f f i n I s l a n d . J o u r n a l of G l a cj. elegy, V o l . 9, No. 55, p. 49-54, 45 APPENDIX Some c o n s i d e r a t i o n s on the s e l e c t i o n and use of t h e r m i s t o r s f o r the purpose of making a b s o l u t e temperature measurements i n snow or i c e . 46 1 INTRODUCTION In g e n e r a l , t h e r m i s t o r s , when used as temperature measuring d e v i c e s , are used s i n g l y or i n p a i r s i n some impedance net, t h a t net being d r i v e n by some power supply, and a l s o having an output, which i s monitored by a d e t e c t o r . Frequently the t h e r m i s t o r i s placed at the end of a transmission l i n e ( F i g . 1). Errors i n p r e c i s i o n and accuracy enter from numerous sources: from the t h e r m i s t o r i t s e l f , from the power supply or i t s c o n f i g u r a t i o n , from the the d e t e c t o r , impedance net or t r a n s m i s s i o n l i n e . The measurement of temperature can only be as good as the p r e c i s i o n standard a g a i n s t which the t h e r m i s t o r was c a l i b r a t e d . I f i t has been assumed t h a t the t h e r m i s t o r c h a r a c t e r i s t i c w i l l f i t a t h e o r e t i c a l curve, then the accuracy can be no b e t t e r than the q u a l i t y of that f i t . Of n e c e s s i t y one must assume that a given t h e r m i s t o r w i l l be s t a b l e over the long term, although i t i s known that t h i s i s not n e c e s s a r i l y the case; environmental f a c t o r s such as s t r e s s or moisture may a f f e c t a t h e r m i s t o r ' s behaviour, and over a p e r i o d of time a thermistor»s s t u c t u r e may change. Preaging can reduce these e f f e c t s , however they cannot be e l i m i n a t e d . I f a t h e r m i s t o r i s r e c o v e r a b l e f c r r e c a l i b r a t i o n a f t e r i t s p e r i o d of use, i t may be p o s s i b l e to monitor some cf these e f f e c t s . P O W E R S U P P L Y V V I N T E R F A C E CB R I D G E ) D E T E C T O R T R A N S M I S S I O N L I N E F I G U R E 1: G E N E R A L I Z E D B R I D G E 48 I t i s the i n t e n t of t h i s paper t c examine i n s t r u m e n t a l parameters, and t o d i s c u s s how they may be used or determined, i n order to optimize the p r e c i s i o n with which the r e s i s t a n c e cf a given t h e r m i s t o r i n a f i x e d s i t u a t i o n , can be determined. These parameters i n c l u d e : t h e r m i s t o r s e l f - h e a t i n g , nominal t h e r m i s t o r r e s i s t a n c e , t r a n s m i s s i o n l i n e r e s i s t a n c e and reac t a n c e , impedance net or brid g e c o n f i g u r a t i o n , type c f bridge power supply, and g u a l i t y of the d e t e c t o r . T h i s examination a p p l i e s e q u a l l y to both c a l i b r a t i o n systems and f i e l d s i t u a t i o n s s i n c e both c o n d i t i o n s can be completely d e s c r i b e d . 2 THERMISTOR CHARACTERISTICS 2.1 THERMISTOR SEIF-HEATIUG I f we assume t h a t a t h e r m i s t o r has s p h e r i c a l symmetry, and t h a t the ambient temperature of the surrounding i c e (or snow) i s constant at T, then the steady s t a t e s o l u t i o n of the heat equation shows t h a t the temperature of i c e c l o s e to the th e r m i s t o r can be no g r e a t e r than T + 4 , k ? r < D x c e where P i s the power d i s s i p a t e d i n the t h e r m i s t o r , K ± c e i s the thermal c o n d u c t i v i t y of i c e (assumed t c be constant with T) and r i s the r a d i u s from the c e n t e r of the t h e r m i s t o r . Let 49 AT. = -7—7^ (2) i c e 4frk. r i c e and l e t AT+T be the temperature as i n d i c a t e d by the r e s i s t a n c e c f the t h e r m i s t o r . I t f o l l o w s t h a t AT > AT. I . = . , P (3) i c e ' mm 4 7 r k. r . i c e min where r m l n i s the r a d i u s of the i c e c l o s e s t t c the t h e r m i s t o r ; i n e f f e c t the r a d i u s of the t h e r m i s t o r . Owing to the f a i l u r e of our s p h e r i c a l approximation i n (1) and t c the great v a r i e t y i n s t r u c t u r e and shape of t h e r m i s t o r s , i t s h a l l be assumed here t h a t the r e l a t i o n i n (3) i s an e g u a l i t y , where r m-£ n must now be a t y p i c a l r a d i u s . Manufacturers of t h e r m i s t o r s g e n e r a l l y provide f i g u r e s f o r t h e i r t h e r m i s t o r s t h a t i n d i c a t e the a b i l i t y of the device to d i s s i p a t e power i n t o the surrounding medium. T h i s • D i s s i p a t i o n C o n s t a n t 1 which s h a l l be c a l l e d E c , i s s p e c i f i c a l l y d e f i n e d as the amount of power r e g u i r e d to r a i s e the temperature of the t h e r m i s t o r 1°C above the ambient temperature. More c o n v e n i e n t l y from (3), i n i c e D = 4TTk. r . (5) c i c e mm 50 Equation (4) c l e a r l y depends on the environment of the t h e r m i s t o r . Manufacturer's f i g u r e s are u s u a l l y given f o r the t h e r m i s t o r i n s t i l l a i r . In i c e Dc i s at l e a s t a f a c t o r of three g r e a t e r [Fenwal D-1; Eg. ( 5 ) ] , and although i n c r e a s e s i n It w i l l improve the measurement p r e c i s i o n , a f a c t o r cf three i n D c r e s u l t s i n no more than 1.5dB improvement i n p r e c i s i o n . Hence f o r the examples used here, s i n c e D c i s g e n e r a l l y d i f f i c u l t t o measure or c a l c u l a t e , manufacturers' f i g u r e s w i l l be used. 2.2 THERMISTOR SELF-HEATING WITH WATER LAYER I f the temperature of the i c e i s s u f f i c i e n t l y c l o s e to the melting p o i n t that before reaching steady s t a t e the temperature of the t h e r m i s t o r reaches the melting p o i n t a l a y e r of water w i l l form arround the t h e r m i s t o r . Since the thermal c o n d u c t i v i t y of water i s much s m a l l e r than that of i c e , the t h e r m i s t o r w i l l tend to s e l f - h e a t g r e a t l y . T h i s e f f e c t has been demonstrated i n attempts t c measure the thermal c o n d u c t i v i t y of i c e by using a thermal d r i l l cable as a l i n e heater, and attempting t c observe the l o g a r i t h m i c r i s e cf temperature with time. I f too much power i s s u p p l i e d to the c a b l e a water l a y e r forms before the l c g a r i t h m i c approximation becomes v a l i d . In another case, i n temperate i c e , the i c e cannct conduct power away from the t h e r m i s t o r . The o n l y a v a i l a b l e s i n k i s i n 51 the i c e melt, hence steady s t a t e w i l l never be approached. However a f t e r s u f f i c i e n t time the ice/water boundary should be s u f f i c i e n t l y d i s t a n t and l a r g e , t h a t i t may be modelled as s t a t i o n a r y . In t h i s case D = 4 TT k r . (6) c water mm v ' a l s o i f too much power i s a v a i l a b l e , induced water c u r r e n t s may cause f l u c t u a t i o n s i n the t h e r m i s t o r temperature. Any system which must c o n s i d e r t h i s case should be designed so that t h i s e f f e c t i s too s m a l l to be measurable. 2.3 THERMISTOR RESISTIVITY GRADIENT Th e r m i s t o r s have negative thermal c o e f f i c i e n t s of r e s i s t i v i t y , which s h a l l be c a l l e d Bp. For most t h e r m i s t o r s RT - -0.05°C - 1 (7) and t h i s f i g u r e w i l l be used i n a l l of the examples i n t h i s paper. 2.4 TIME CONSTANTS The 'time constant' of a t h e r m i s t o r , as d e f i n e d by some manufacturers i s the time r e q u i r e d f o r a t h e r m i s t o r tc change i t s temperature 63% of the amount of temperature change of a value impressed upon i t i n a st e p change [Fenwal EMC-5]. For 52 a given t h e r m i s t o r t h i s may vary from f r a c t i o n s c f seconds to minutes [Fenwal D-1] as a f u n c t i o n of environment. For steady s t a t e temperature measurements a more u s e f u l time constant would be the time r e g u i r e d f o r a t h e r m i s t o r t c reach 63% of #T above T, from the i n i t i a t i o n c f power, but because of the e f f e c t s of environment on Dc and hence AT, and a l s o c o n s i d e r a t i o n o f the f a c t t h a t the twc co n s t a n t s as defin e d here are probably c l o s e l y r e l a t e d , the l a t t e r time constant i s almost c e r t a i n l y i n d e t e r m i n a b l e , e s p e c i a l l y when the t h e r m i s t o r would be deployed i n a bore ho l e i n i c e . The best t h a t should be s a i d about a t h e r m i s t o r being used to measure a steady s t a t e temperature i s that the temperature i t measures i s between T and T+AT, both as d e f i n e d b e f o r e . I t i s necessary then t h a t the al l o w a b l e e r r o r due to s e l f - h e a t i n g must be the f u l l value of T, and that i n most cases t h i s w i l l not be reduced. 3 BRIDGES AND BRIDGE POWER SUPPLY. 3.1 POWER SUPPLY TYPES In t h i s paper only two kinds of power supply w i l l be considered. They are f i r s t a D.C. supply, and secondly an A.C. supply of angular freguency u . Further i n t h i s paper they w i l l be shown to be both o p t i m a l when compared tc pulsed 53 supplies. 3.2 BRIDGE TYPES The simplest type of resistance measuring device consists of a current source driving the unknown resistance. A voltmeter then measures the voltage drop accross the resistance (Fig. 2A), A measure of the signal available i s dV/dR. In t h i s case S i - * <« Thermal noise voltage i s proportional tc provided we assume that the thermistor i s an i d e a l Johnson ncise generator. Signal to noise r a t i o i s thus proportional to i//R. A l l analog ohmmeters and d i g i t a l chmmeters work i n t h i s fashion. The d i f f i c u l t i e s involved i n taking t h i s approach i s that with analog meters, they r a r e l y have enough dynamic range cr precision to be useful, and with d i g i t a l meters long term l i n e a r i t y and short term thermal s t a b i l i t y are generally not good enough so that o v e r a l l accuracy approaches the resolving c a p a b i l i t y of the instrument. This i s particulary the case when d i f f e r e n t instruments are used for c a l i b r a t i o n and f i e l d measurements. Null detectors have the advantage that they do not require the dynamic range or l i n e a r i t y of the •chmmeter* type cf instrument. However, they may s t i l l be thermally 54 FIGURE 2: BRIDGE CONFIGURATIONS 55 s e n s i t i v e , ana they cannot be used f o r ' i n s t a n t * measurements. T h i s i s a problem i f continuous measurements i n time ever a l a r g e temperature range are r e g u i r e d . For making s i n g l e measurements of steady s t a t e temperatures, n u l l d e t e c t o r s are we l l s u i t e d . F i g u r e s 2B and 2C show two p o s s i b l e c o n f i g u r a t i o n s . The c h i e f d i f f i c u l t y i n implementing a system g e n e r a l i z e d by the type i n Fi g u r e 2E l i e s i n p r o v i d i n g an accurate low impedance v o l t a g e r e f e r e n c e v B/2 e x a c t l y h a l f of the primary b r i d g e supply (In a l l cases v B r e f e r s t o the B.M.S. V o l t a g e ) . F i g u r e 2C r e p r e s e n t s a Sheatstcne bridge with the f o u r arms e q u a l . In F i g u r e 2B v dV = _B dR 4R and noise voltage i s times s m a l l e r than i n case A. S i g n a l to n o i s e v o l t a g e r a t i o i s then p r o p o r t i o n a l t c VB / 8T 3 \" I f power d i s s i p a t e d i n the t h e r m i s t o r i s the same i n both cases V 2 i 2 R = 7 J - ( 1 0 ) and s i g n a l to nois e r a t i o s may be compared. I t f o l l o w s that S/U v o l t a g e r a t i o f o r case A i s ^2 times g r e a t e r than f o r case B, and i s two times g r e a t e r than f o r case C ( F i g . 2C). 56 D i f f i c u l t i e s i n c o n s t r u c t i n g r e l a t i v e l y n o i s e l e s s c u r r e n t and voltage sources would l i k e l y counter the advantages c f case A or B over case C. For t h i s reason, and f o r ease i n c a l c u l a t i o n s , case C w i l l c o ntinue to be used. 3.3 BRIDGE AND DETECTOR RESOLUTION Let A T * be the d e s i r e d temperature r e s o l u t i o n . Then M = A T * R T (11) where A R i s the necessary r e s o l u t i o n i n terms of r e s i s t a n c e . From (9) and (11) I?- = — * — (12) A V A T * R T where A v i s the r e q u i r e d v o l t a g e r e s o l u t i o n cf the d e t e c t o r . Johnson noise power from the bridge i s i d e a l l y Pn = Y= 4kTB (13) where V n i s n o i s e v o l t a g e , k i s Boltzmann's constant, T i s i n degrees K e l v i n , and B i s the bandwidth of the d e t e c t o r . A convenient measure of the q u a l i t y of the d e t e c t o r may be d e f i n e d as S = ^ = n o i s e f i g u r e (14) n V n where AV now r e p r e s e n t s the d e t e c t o r ' s best r e s o l v i n g a b i l i t y R e c a l l from (10) that 57 V P = -2— 4R From (10), (12), (13), and (14) s 2 PR„ A T * = (1 6 ^ X15) T But from (4) P = A T D (16) c A minimum of the sum of A T and A T * occurs at A T * A M - T T - = A T (17) It follows that AT = ( ^ 4 ^ ( 1 8 ) c T AT* = (32XT)1/3 (19) c T AT + AT* = ( 1 0 8 S n 2 k T B ) l / 3 ( 2 Q ) D K m c T Equations (18), (19) and (20) are useful for determining the optimum resolution of any system as the parameters approach the i d e a l . More r e a l i s t i c a l l y i t would be convenient to have an expression for the optimum in terms of the detector resolution Ay. From (13), (14), (18), (19) and (20) 58 AT = ( A V2 1/3 (21) RB CR T AT* = ( 8AV2 1/3 (22) AT + AT* = ( (23) From (21), (22) and (23) d e t e c t o r parameters and power l e v e l s may be o p t i m i z e d , and o v e r a l l a c c u r a c i e s known. Consider f o r example a t h e r m i s t o r with D c = Imwoc - 1 which s h a l l be operated a t 10K^. Suppose our d e t e c t o r has an i n p u t noise v o l t a g e of ivv . Then Here s i g n a l power has been assumed to be egual t o n o i s e power, G e n e r a l l y i t would be d e s i r a b l e t o have s i g n a l t c noise r a t i o s cf a t l e a s t 10dB. Then 3.4 D.C. BRIDGES I f a D.C bridge i s used with a good d e t e c t o r [ e g . ANALOG DEVICES chopper amp. Model 261K] i t would have input n o i s e on AT + AT* = 0.001°C AT + AT* - 0.0023°C 59 the order of 1vv P-P with a bandwidth to 10Ez. Since a reasonable response time i s d e s i r a b l e , a bandwidth c f 5 Hz i s minimal. Input n o i s e voltage d r i f t of 0.1VV°C - 1 would boost e f f e c t i v e input v o l t a g e noise to about 3.5W. T h i s would boost a usable A v to 12PV, and i n our example AT + AT* = 0.0054°C T h i s may be co n s i d e r e d a p r a c t i c a l l i m i t i n g accuracy f o r a t h e r m i s t o r with D c = 1 mwoc - 1 and B = 10Kfi, when d r i v e n by a D.C. system. Other d i f f i c u l t i e s i n v o l v e d i n using a D.C. system a r i s e from thermal voltage o f f s e t s , and from the p r o x i m i t y of 50 Hz or 60 Hz sources. The f i r s t may be helped by s e l e c t i n g a chopper s t a b i l i z e d d e t e c t o r or s i m i l a r d e t e c t o r designed f o r thermal immunity. The second, which may be l a r g e enough to obscure measurements i n s p i t e of a sharp r o l l o f f should be helped by using good s h i e l d i n g and grounding (the most l i k e l y mechanism would be that the t r a n s m i s s i o n l i n e , a c t i n g l i k e an antenna, would provide s u f f i c i e n t common mode s i g n a l to overload the d e t e c t o r i n p u t ) . 3.5 ft, C. EBIDGES I t i s p o s s i b l e t o get a m p l i f i e r s which operate at audio f r e q u e n c i e s , t h a t have c o n s i d e r a b l y l e s s i n p u t noise voltage per u n i t bandwidth than D.C. a m p l i f i e r s . Detectors, e i t h e r phase-locked, or simple, are e a s i l y designed. Bandwidths may 60 be l i m i t e d to l e s s than 100 Bz, which w i l l keep input noise voltage w e l l below the best a v a i l a b l e D.C. d e t e c t o r s . For systems where extremely high p r e c i s i o n i s necessary, the designer may c o n s i d e r using an a.C. voltage source t c feed h i s b r i d g e . However, new design problems accompany the c h o i c e of an A.C. source. The o p e r a t i n g frequency should be kept as low as p o s s i b l e . The reason i s tw o f o l d : f i r s t , s i n c e a lew bandwidth i s d e s i r a b l e , a low c e n t e r frequency would minimize the need fo r a l a r g e Q; second and predominant, the e f f e c t s of s t r a y reactance i n the b r i d g e , and p a r t i c u l a r l y i n the t r a n s m i s s i o n l i n e to the t h e r m i s t o r would be minimized. Belcw 100 Hz f l i c k e r noise predominates and a l l the d i f f i c u l t i e s c f D .C. b r i d g e s ensue. 3.6 OPTIMUM BRIDGE SUPPLIES I t i s p o s s i b l e to use a pulsed power supply or seme ether form of i n t e r m i t t e n t supply. However i n no case can one improve on e i t h e r a pure D.C. or pure sine wave source i f one c o n s i d e r s the s i g n a l to noise r a t i o . T h i s can be shown as f o l l o w s . S i n c e a l l measurements must be of f i n i t e d u r a t i o n we can assume t h a t the power supply p a t t e r n r e p e a t s r e g u l a r l y . Consider i t s F o u r i e r decomposition. The maximum s i g n a l to noise r a t i o occurs when a l l the s i g n a l power i s confi n e d to the minimum bandwidth; i . e . i n one component. Any attempt cf 61 spreading the s i g n a l power among more than one component n e c e s s a r i l y i n c r e a s e s the r e q u i r e d bandwidth of the d e t e c t o r and thus i n c r e a s e s n o i s e . Hence a s i n g l e component, sine wave or pure D.C. i s o p t i m a l . 4 TRANSMISSION LINE EFFECTS 4.1 TRANSMISSION LINE RESISTANCE Presumably i t i s p o s s i b l e t c determine the r e s i s t a n c e of the t r a n s m i s s i o n l i n e being used, or at l e a s t get an estimate well w i t h i n the r e q u i r e d accuracy. T h i s may then be c o r r e c t e d f o r when determining the a c t u a l t h e r m i s t o r r e s i s t a n c e . I f a t h r e e or f o u r wire t r a n s m i s s i o n l i n e i s used, the problem can be e l i m i n a t e d e n t i r e l y . 4.2 TRANSMISSION LINE REACTANCE In an A.C. b r i d g e , i t i s d e s i r a b l e to know what the maximum t r a n s m i s s i o n l i n e c a p a c i t a n c e C i s which w i l l s t i l l permit a nul l of amplitude v. I f C i s s u f f i c i e n t l y s m a l l , then i t s e f f e c t i s to phase s h i f t the c u r r e n t , amplitude changes being second o r d e r . The c o n s t r a i n t on C i s a)RC < ^ (24) B I f the d e t e c t o r i s phase-locked then 62 ^ o b s e r v e d — / / R j ioC a c t u a l (25) To a f i r s t approximation AR = R - R = f ( c o R C ) 2 (26) a c t u a l o b s e r v e d 2 ' provided the transmission l i n e i s short (this error w i l l usually be s u f f i c i e n t l y small that an estimate of C w i l l y i e l d a correction AR of s u f f i c i e n t accuracy). 5 EVALUATION OF AVAILABLE SYSTEMS (DMM.S) On the basis of (16) and the r e l a t i o n cf A T * tc P (from (13) (14) and (15)) AT* = (27) Rt7PR~ It i s possible to evaluate and compare measurement systems for a given R and Dc. The following d i g i t a l multimeters have been evaluated as thermistor measurement systems with respect to thermistor selfheating, displayed p r e c i s i o n , guaranteed accuracy, each when used with a FENHAL GB34P2 thermistor - 10K < R < 15K, D c = Imwoc-i. 63 FLUKE MODEL 8 100A FLUKE MODEL 8000A DANAMETER 2000 DATA PRECISION 245 SYSTRON DONNER 7205 SYSTRCN DONNER 7050 SYSTRON DONNER 7005 TABLE 1: DIGITAL MULTIMETERS Table Two l i s t s pertinent data. CONCLUSION Present technology can e a s i l y y i e l d a device for measuring thermistor resistances within 0.01°C, however the value of achieving t h i s resolution of absolute temperature measurements can be guestioned. In deep boreholes, where the thermistor resistances are used to i n f e r both absolute temperatures and temperature gradients, the thermistors are not recoverable and long term s t a b i l i t y l i m i t s the accuracy possibly tc only 0.4°c (Muller S Stolton, 1953) although 0.08<>c i s more common [Fenwal- TI-1] and0.02<>C i s possible (Beck, 1956). 64 Ar Errors in Temperature Measurement vs..Power to a FENWAL GB34P2 Thermistor, for Seven Digital Voltmeters The diagonal line represents thermistor self-heating. The thick error bars represent the meter resolution. The thin error bars represent the meter accuracy. The numbers in circles indicate the points plotted from Table 2. FIGURE 3: DIGITAL \"MULTIMETER ERRORS 65 UNIT RANGE POWER AT AT* ( 1 ) AT* (2) FLUKE 8100A 1 2 K f i 120yW 0 . 1 2°C 0.002OC 0.02OC CD 1 2 0 Kfi 13yW 0.013«c 0 . 0 2 « C 0 . 2 ° C DATA PRECISION 245 20Ktt 1, 3mW 1.3«c 0 . 0 0 2 « C 0. 1 °C DANAMETER 2000 ® 20Kfi 140yW 0 . 1 4 ° C 0.02OC 0. 04 OC FLUKE 8000A 20Kfi 120yW 0.120C 0 . 0 2 ° C 0.06OC ® SYSTRON DONNE R 7050 150Kfi 1. 2yW 0 . 0 0 1 ° C 0. 2°C 0 . 2 8 ° C SYSTRON DONNER 7205 130Kfi 1.2yW 0 . 0 0 1 « C 0.002OC 0 . 0 1 ° C 13KR 120yW 0 . 1 2 ° C 0 . 0 0 0 2 « C 0 . 0 0 1 ° C SYSTRON DONNER 7005 ® 130Kfi 30yW 0 . 0 3 ° C 0 . 0 2 « C 0 . 0 4 O C 13Kfi 3mW 30C 0 . 0 0 2 ° C 0.004OC TABLE 2: DIGITAL HULTIKETER ERRORS Data l i n e s followed by numbers in c i r c l e s are graphed for comparison in Figure 3, AT*(1) i s the re so lu t ion determined by the number of ava i lab le d i g i t s . AT* (2) i s the e f fec t ive guaranteed accuracy of the meter. In shallow experiments such as snow pack s tudies , or permafrost de tec t ion , f ine temperature re so lu t ion i s not required s ince i n one case the temperature var ia t ions are gross and nonl inear , and i n the l a t t e r the experimenter general ly wishes only to detect subfreezing temperatures, not to estimate them. The greatest value in achieving f ine resolut ion of thermistor res i s tances l i e s f i r s t i n thermistor c a l i b r a t i o n s (it i s poss ible to achieve 0.03$ accuracy ea s i ly and rapidly) and in experiments where temperature transients are measured. Thermistor measurement technique i s an important considerat ion in any temperature measurement system where 66 optimum accuracy i s r e q u i r e d . E r r o r s due to t h e r m i s t o r s t a b i l i t y , r e f e r e n c e temperatures, and measurement are comparable i n magnitude. 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T e c h n i c a l Note: [ T h e r m i s t o r ] S t a b i l i t y and R e l i a b i l i t y C h a r a c t e r i s t i c s . FENWAL ELECTRONICS, INC., TC-1. T e c h n i c a l Note: C o n s i d e r a t i o n s i n the t e s t i n g cf t h e r m i s t o r s . FENWAL ELECTRONICS, INC., TD-2, EM-34/ R e v i s i o n 2. "@en ; edm:hasType "Thesis/Dissertation"@en ; edm:isShownAt "10.14288/1.0052944"@en ; dcterms:language "eng"@en ; ns0:degreeDiscipline "Geophysics"@en ; edm:provider "Vancouver : University of British Columbia Library"@en ; dcterms:publisher "University of British Columbia"@en ; dcterms:rights "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en ; ns0:scholarLevel "Graduate"@en ; dcterms:title "Ultra high frequency radio echo sounding of glaciers"@en ; dcterms:type "Text"@en ; ns0:identifierURI "http://hdl.handle.net/2429/19268"@en .